Conventionally, there has been proposed a method for estimating a load acting on a tire, in which a plurality of piezoelectric elements for detecting a change of the load in a circumferential direction and a plurality of piezoelectric elements for detecting a change of the load in a width direction, the load acting on the tire during traveling of a vehicle, are arranged on an inner surface side of a tire tread, and in which a ground contact area of the tire is calculated on the basis of a ground contact length in the circumferential direction of the tire, which is detected from the change of the load in the circumferential direction and a ground contact width in the width direction of the tire, which is detected from the change of the load in the width direction, an air pressure of the tire is measured, and on the basis of the measured air pressure and the calculated ground contact area, the load acting on the tire is estimated (see PTL 1, for example).